chromium/third_party/blink/web_tests/external/wpt/css/selectors/featureless-001.html

<!doctype html>
<meta charset="utf-8">
<title>Verifying that :host works as a selector at all</title>
<link rel="help" href="https://drafts.csswg.org/selectors-4/#featureless">
<link rel="match" href="/css/reference/ref-filled-green-100px-square-only.html">
<!-- All these tests use a shadow host as the featureless element,
  so we need to make sure that the impl actually supports the
  necessary functionality. -->

<p>Test passes if there is a filled green square.</p>
<div class=host>
    <template shadowrootmode="open">
      <style>
        div {
          width: 100px;
          height: 50px;
        }
        .red { background-color: red; }
        .green { background-color: green; }
        :host div.red {
          /* Make sure :host matches the host element... */
          background-color: green;
        }
        div > div.green {
          /* And make sure *other* selectors *don't* match it. */
          background-color: red;
        }
      </style>
      <div class=red></div>
      <div class=green></div>
      <slot></slot>
    </template>
    <section></section>
</div>